The moon itself does not glow, but reflects light from the sun. Whether the author was making this comparison or that the glow was soft and silvery like we attribute to the moon, is hard to say.

Abilities

Silver Phantoms are known to be the fastest and highest-flying dragons around, save, perhaps, for the Guardian dragons. They can also fly at the highest altitude, so high the air starts to thin. They are "very, very rare and very, very dangerous".

Silver Phantoms also have excellent sight, hearing, and sense of smell. The 'Dragon Key' at the back of The Incomplete Book of Dragons also indicates Silver Phantoms can produce electricity, and shoot lasers or missiles (unclear which).

Silver Phantoms also produce a lovely, lethal fire:

"And as the dragon screamed, it poured out a jet of bright blue flames that blasted the trees in front of it, burning the leaves as bright as green stars before they dropped to the ground in powdery black smithereens."